This proposal is concerned with the origins and therapy of HER driven tumors. One aspect of our work is to develop an understanding of transcriptional processes involved in phenotype. Tumor pathogenesis involves adaptive and allelic changes as tumors evolve to a tumor resistance phenotype, a state often accompanied by abnormalities of DNA repair. Our studies, as well as those of others, have shown that both HER2/HER4 and HER2/EGFR heteromers influence levels of the transcription co-activator YAP (Yes-associated protein). YAP associates with HER4, which phosphorylates it. The phosphorylated YAP complex is transcriptionally active. Because YAP is a transcriptional co-activator, it reciprocally influences the targeted antibody resistant phenotype, since YAP increases expression of ligands for HER members. The phenotypic changes in HER expressing cells caused by ligands such as amphiregulin are adaptive adjustments. YAP has also been identified as relevant to pluripotency of stem cells. We and others found that increased HER signaling, in general, enhances DNA repair processes, even in the face of genotoxic injury, making cells resistant to certain cell death processes. Antibodies to HER2 reverse phenotype and limit HER2 mediated signaling induced resistance to DNA damage. Some elements of DNA repair are known in detail, and our work has led us to identify SUN protein nucleated repair nodes on the inner surface of the nuclear membrane, which are involved in this process. This work extends our efforts of targeted therapy of HER tumors and how to overcome therapeutic resistance. Our work will study post translational changes of YAP in HER transformed cells and elements of the YAP pathway in BT-474 breast tumor cells (HER2+, HER4+, EGFR+) that are sensitive and resistant to targeted HER therapy in order to ascertain the role of the YAP pathway in resistance to targeting small kinase inhibitors or targeted mAb. Secondly, we will examine some defined aspects of how HER signaling alters DNA repair processes. We will employ the amino terminus domain of SUN2 in yeast cells exposed to genotoxic signals to determine if this fragment limits repair. How SUN function is affected by HER mediated transformation of mammalian cells will be ascertained in vitro using BT-474 cells and a metastatic subline BT-474M, and in vivo using novel mouse strains.